My mail gets filtered by qmail's rblsmtpd performing reverse DNS lookups at inputs.orbz.org, outputs.orbz.org, dialups.relays.osirusoft.com, and spews.relays.osirusoft.com. Then, after all that, I run my own block list (via DNS, of course) that is self-maintaining. All mail that arrives at one of my several spamcatcher addresses results in the delivering IP address being blocked for 30 days, and the offending message is archived for 90 days. It will also receive bounced mail from any local user, and block the IP address that delivered it into my domain.

This contraption is based upon qmail and rbldns, with lots and lots of perl to handle the self-maintaining list. Eventually I'll get back to the perlTk management interface for the thing... not that I've needed it in the past several months during which it's been keeping my inbox as clear as a mountain stream. :-)